the M16 rifle: M16A1, M16A2, M16A3, M16A4 (USA)

The story of the M16 rifle is one of the most turbulent and controversial episodes in the whole history of US small arms.

The M16A1 rifle (top) and M16A2 rifle (bottom)

The M16A1 rifle (top) and M16A2 rifle (bottom)


It was hastily adopted as an interim measure, but soldiered on to eventually see more than 50 years of active service – continuing to this day. Its early days were full of controversy and scandal and its present day is full of competition. It does appear though, that this weapon will serve with American armed forces and abroad for at least several years (if not decades) more.
Today it can be considered an adequate military rifle with a good service record. Clearly, it is far from being ideal, but no rifle in the world is perfect. We will have to wait and see if the American armed forces will eventually produce a better rifle and / or cartridge in the forseeable future.

Origins of the M16.

The origins of the M16 rifle lay in the research conducted soon after the Korean war by the Operations Research Office (ORO), founded at Hopkins University and sponsored by the US Army. Among the reports produced by the ORO, two are more significant in this respect, the so called Hall and Hitchman reports.
One report stressed the fact that most hits achieved by soldiers in battle, were made at relatively short ranges (within 300 meters) and at random and not aimed. This significantly undermined the obsession for long-range aimed fire, up until then promoted by the Army (to the discomfort of the UK and Europe.
The Second report suggested that the most effective way to increase the probability of hits in battle is to fire multiple small caliber, high velocity projectiles with controlled dispersion. This is instead of one, relatively heavy and large projectile as used in conventional rifles at the time. The latter concept initiated the so called “Project SALVO”, which was conducted between 1952 and 1957 to develop the proper concept of a new, small bore military rifle.
There were several basic ideas to follow up on, including different projectile types (standard bullets or small, arrow-like finned projectiles known as “flechettes”, multiplying from a single round to achieve a ‘shotgunlike’ effect) or in rapid bursts of several single rounds.
Eventually, the army selected the concept of a weapon firing controlled bursts of singular flechettes (steel arrows with a body diameter of around 1.5mm) to go ahead, and called this APHHW – All Purpose Hand Held Weapon, later renamed to SPIW – Special Purpose Individual Weapon.

In the meantime, some elements within the Army also sponsored a more conventional approach to the same problem, and ordered the development of conventional automatic rifles, firing ordinary small-caliber bullets.
During the late 1950’s and early 1960’s there was a lot of experimentation and development in regard to new ammunition, optimal calibers and best rifle designs. The problem was, that the widely promoted SPIW program seemed to have no end, and the recently adopted “full-power” 7.62mm M14 rifle faced serious production problems.
Enter Armalite. In the year 1957 The US Army requested the Armalite Division of the Fairchild Aircraft Corp to develop a rifle of .22 caliber, lightweight, select-fire, and capable of penetrating a standard steel helmet at 500 meters.

Eugene Stoner, then a designer at Armalite, began to develop this rifle, based on his earlier design – the 7.62mm AR-10 automatic rifle. At the same time, experts at Sierra Bullets and Remington, in conjunction with Armalite, began to develop a new .22 caliber cartridge, based on the .222 Remington and .222 Remington Magnum hunting cartridges.


The Armalite AR-15 rifle, first prototype

The Armalite AR-15 rifle, first prototype


This development, initially called the .222 Remington Special, was finally released as .223 Remington (metric designation 5.56x45mm). The next year the Army tested new rifles, known as AR-15, and rejected those in favor of the M14.

Feeling that the AR-15 rifle had a poor chance to compete with the recently adopted M14 in the US Military, in 1959 the Fairchild Corp, the parent company of Armalite, sold all rights and manufacturing documentation for this rifle to Colt’s Patent Firearms Manufacturing Company.
Colt had long-time relations with the US Military and a proven track record of selling military guns both in the USA and abroad. They instantly began an aggresive marketing campaign for the new rifle, stressing its accuracy, low recoil and lightweight, modern design. They also rather unfortunately marketed as ‘self cleaning’ and ‘low maintenance’.

The Colt AR-15 rifle, model 001

The Colt AR-15 rifle, model 001


In 1962, US DoD Advanced Research Projects Agency (now DARPA) purchased 1000 AR-15 rifles from Colt. These rifles were sent to South Vietnam for field trials. The same year brought glowing reports back to the states about the effectiveness of the new “black rifle”, used by South Vietnamese forces.


The Colt AR-15 rifle, from US military trials report, 1962

The Colt AR-15 rifle, from US military trials report, 1962

Military service history.

Following the delays in the introduction of the ill-fated ‘next generation’ SPIW system and production troubles with the M14, in 1963 Colt received contracts from the US Government for 85 000 rifles for the US Army (designated as XM16E1) and a further 19 000 rifles for the US Air Force (designated M16).
The USAF M16 rifle was no more than an AR-15 rifle with appropriate markings. The XM16E1 rifle differed from AR-15/M16 by having an additional device, the so called “forward assist”, which was used to manually push the bolt group in place in the case of a jam. The very next year US Air Forces officially adopted the new rifle as M16. At the same time the US Army adopted the XM16E1 as a limited standard rifle, to fill the niche between the discontinued 7.62mm M14 rifle and the forthcoming SPIW system (which in the end, never got past the prototype and trial stages).


The USAF M16 rifle

The USAF M16 rifle, shown with later model 30-rd magazine


The XM16E1 rifle

The XM16E1 rifle


With the rapidly growing presence of US troops in Vietnam, in 1966 the US Government made the first large purchase of Ar-15 / M16 rifles, ordering 840 000 rifles for the US Armed forces, a deal worth almost $92 million. So in 1967 the US Army officially adopted the XM16E1 rifle and redesignated it as the standard ‘US Rifle, 5.56mm, M16A1‘.
During the following few years, there were a number of very negative reports that came from Vietnam. Many of the M16A1 rifles, issued to US troops in Vietnam, severely jammed in combat, resulting in numerous casualties.


The M16A1 rifle

The M16A1 rifle



The M16A1 rifle

The M16A1 rifle from Vietnam war era, showing official markings


There were some causes for the malfunction. First of all, during the introduction of the new rifle and its ammunition into service, the US Army replaced the originally specified Dupont IMR powder with standard ball powder, as used in 7.62x51mm NATO ammunition. The standard ball powder, in conjunction with the ‘direct impingement’ operating system, produced much more fouling, quickly jamming the actions of the M16 unless the gun was cleared well and often by someone trained in the procedure.
It also had a different ‘pressure curve’, resulting in increased stress on operating parts of the gun. This pitifully combined with the fact that the initial M16 rifles were promoted by Colt as being “self cleaning”. This meant that for the sake of economy, no cleaning supplies were procured for the new M16 rifles and no weapon maintenance training was conducted for the troops. As a result, soldiers did not know how to clean their rifles, and had nothing to clean them with.
Another cost-saving measure on the part of the Army was to not bother with the chromium plating of the barrel bore and bolt group, which made these parts much more sensitive to corrosion and rust than the original design.
After several dramatic reports in the US press and a Congressional investigation into the troubles, several actions were taken to remedy the problems.
The 5.56mm ammunition was now loaded using a different powder that produced much less residue in the gun action. The barrel, chamber and bolt of the rifles were again chrome lined to improve corrosion resistance. Cleaning kits were procured and issued to the troops, and specific training programs were developed and have been conducted ever since. The earliest cleaning kits could be carried separately from the rifle only, but since circa 1970, all M16A1 rifles were manufactured with a containment cavity in the buttstock holding the cleaning kit.

At the same time (1970) new 30 round magazines were introduced into service instead of the original 20 round ones, to equal the Soviet and Chinese Kalashnikov assault rifles, which had 30-round magazines from the beginning.
So by the end of US involvement in the Vietnam war, the M16A1 rifle had become more or less mature. It gradually replaced the older rifles in US service, and also influenced the work on small-caliber ammunition and automatic firearms in other countries, including in the USSR.
This work culminated in NATO trials, held in 1977 – 79, with the intent to adopt a small-caliber, high-velocity cartridge to replace the potent, but somehow overly powerful 7.62mm NATO round as the standard infantry rifle ammunition for the whole NATO organization. Not surprisingly, the winner of the trials was the American 5.56x45mm cartridge, although in a version loaded with a Belgian SS109 (US designation M855) bullet, which provided better long-range ballistics and penetration than the original US 5.56mm M193 ammo.

In 1981, Colt developed a variation of the M16A1, adapted for the SS109/5.56mm NATO cartridge, and submitted it to military trials as the M16A1E1. This rifle differed from the M16A1 by having a heavier barrel with faster 1:7 rifling, a different type of rear sights (adjustable for both range and windage), round handguards instead of triangular ones, and by replacing the full-automatic fire mode with 3 round burst to preserve ammunition. It was officially adopted by the US DoD as ‘US Rifle, 5.56mm, M16A2‘ in 1982. Since 1985 and until late 1990s the M16A2 rifle became the primary individual weapon of US infantrymen, in Army and Marine Corps.


The M16A2 rifle with the M203 grenade launcher

The M16A2 rifle with the M203 grenade launcher


The development of the M16 rifle continued. By the mid-1990’s, Colt, at the request of US Special Forces, produced a shorter version of the M16A2, designated M4 carbine.
By 1996, the two latest versions of the M16 appeared, the M16A3 and M16A4. The rare M16A3, which was ordered only by US Navy, was similar to M16A2 except that its trigger unit delivered unlimited automatic fire instead of the 3-round bursts. The M16A4, which in 1997 became the next standard issue rifle in US military, differs from the M16A2 by having a removable carrying handle, with the upper receiver being fitted with a Picatinny-type accessory rail.


The M16A4 rifle, produced by Colt

The M16A4 rifle, produced by Colt. Rifle is in basic configuration, with detachable carrying handle


The M16A4 rifle, produced by FN

The M16A4 rifle, produced by FN. Rifle is equipped with the RIS forend and with the folding back-up iron sight (BUIS)


The rule of the M16 as a general-issue rifle within the US Armed forces gradually ended by 2010-2015, as it was replaced in front-line units by more compact and lighter M4 and M4A1 carbines. It was, and still is widely used by US Law Enforcement agencies, either in military form (for example, the LAPD had some M16’s, retired from the Army), or in ‘civilian’ semi-automatic form only. The civilian semi-automatic version of this rifle is designated AR-15.

The AR-15 style rifles are made in the USA by many dozens of companies, large (such as Colt, FN or Ruger) or small. Produced in countless versions and many calibers, they literally dominate semi-automatic civilian rifle segment in USA.

M16-type rifles are also manufactured outside the USA, most notably in Canada, by Diemaco Co (now Colt Canada). China also makes some AR-15 type rifles at the NORINCO state factories, known as CQ. Other former licensed manufacturers of the M16 were Philippines, South Korea and Singapore. M16 rifles are used by many foreign military groups, most notably the British SAS, who apparently preferred the M16 over the infamous L85A1 rifle – and why wouldn’t they.
At the present time almost all of the initial flaws of the M16 have been removed and it is considered among the best assault rifles in the world. While its reliability in harsh conditions cannot match that of its main rival, the Kalashnikov AK-47 and AK-74, it arguably rivals it in terms of accuracy. It is still quite a reliable weapon, especially when well maintained. It is also comfortable to fire and has good accuracy.

Flexibility of construction.

One of the key advantages of the Stoner design, that must be especially stressed, is its extreme flexibility of construction. At the present time the interchangeable complete upper receiver assemblies (“upper” for short) are available in various barrel lengths and profiles (from 7 to 24 inches long, slim and heavy), and in dozens of rifle and pistol calibers: (from tiny but fast .17 Remington and up to monstrous .458 SOCOM, and from .22LR and 9mm Luger up to mighty .50AE). Special, manually operated single-shot uppers are commercially available in the extremely powerful .50BMG (12.7x99mm) caliber.
Various “lower receiver” assemblies offer a broad variety of trigger units, buttstocks and other options. This advantage is viable for both military (but mostly Special Ops), Law Enforcement, and civilian applications, as it allows the tailoring of any particular AR-15 type rifle to the current situation and tactical needs.

Technical Description

The original AR-15 rifle and every rifle from the M16 family is a gas operated, selective fire, magazine fed weapon. Generally civilian AR-15 type rifles are semi-automatic only.

The heart of the M16 rifle is the direct gas impingement system, developed by Eugene Stoner in the early 1950’s. This system doesn’t use a gas piston and rod to propel the bolt group back after a shot is fired as is conventional with gas operation. Instead, the hot powder gases are fed directly from the barrel and down a stainless steel tube into the receiver. Inside the receiver, the rear end of the gas tube enters into the ‘gas key’, a small attachment on the top of the bolt carrier. The hot gases, through the gas key, enter the hollow cavity inside the bolt carrier, and expand there. There they act against the bolt carrier and the collar around the bolt body. The pressure of the gases causes the bolt carrier to move back against the initially stationary bolt. The linear rearward movement of the carrier is initially transferred into the rotation of the bolt, via the cam slot in the bolt carrier and the cam pin, attached to the bolt, that follows the slot.

As soon as the bolt is rotated to unlock from the barrel, the bolt group continues its rearward travel under the inertia and the residual pressure in the barrel, extracting the spent case and compressing the buffer return spring, located in the buttstock. The forward movement of the bolt group first strips a fresh cartridge from the magazine and, in the final stage of the movement, rotates the bolt to lock into the barrel extension.

The bolt has 7 radial locking lugs and an eighth lug is located on the extractor claw. Since the introduction of the XM16E1 rifle, the forward assist device is used on all military and most civilian AR-15 type rifles. This device consists of a spring-loaded button with internal claw, that engages the serrations on the right side of the bolt carrier to push it forward. This is in the case that the pressure of the return spring is insufficient to do so (for example, due to fouling inside the receiver or chamber).
The rifle will not fire unless the bolt is locked and the bolt carrier is in its forwardmost position. Another feature of the AR-15 type rifles is the bolt catch device, that locks the bolt group in the open position when the last round is fired. To release the bolt group one must push the button, located at the left side of the receiver, above the magazine. The ‘T’-shaped cocking handle is located at the rear of the receiver, above the buttstock, and does not reciprocate when the gun is fired.

The trigger/hammer group is basically similar to the one found in the M1 Garand rifle. It actually traces its roots back to the early 1900’s, when the great John M. Browning developed his famous Auto-5 semiautomatic shotgun. This essentially consists of a hammer, a trigger, a disconnector, a full auto sear and some springs. The fire selector / safety switch is located at the left side of the receiver, above the pistol grip, and is easily operated by the right hand thumb. This switch has 3 positions: ‘Safe’, Semi-auto’ (single shots) and ‘auto’ (fully automatic on M16A1 and M16A3 rifles or 3 round burst on M16A2 and M16A4). In the latter cases, the trigger unit also includes a ratchet device to count the shots fired.

The ejection port is located at the right side of the receiver, and is closed by the spring-loaded dust cover, which automatically pops open when the bolt carrier is pulled back. The M16A2 also features a spent case deflector – a triangular bulb on the receiver, just behind the ejection port, that allows the gun to be safely fired left-handed.
The M16 is fed using box magazines. The earliest magazines were made from aluminum and held 20 rounds. Circa 1970 the new, 30 round magazines were introduced into service and these magazines are still in service today.

An extremely wide variety of magazines is available on the commercial marked, starting from the ‘US post-ban’ 5 and 10 round magazines, and up to the 40-round box, 90-round helical, 100-round dual drums (Beta-C) or 120-round single drums.

The receiver is made from aluminum alloy, and consists of two parts – the ‘upper receiver’ and ‘lower receiver’ (usually referred to simply as ‘upper’ and ‘lower’). Most receivers are made from machined aluminum, but some commercially available receivers are made from aluminum castings with final drilling and machining. The upper and lower receivers are linked by two cross-pins – one at the front (pivot pin), and one at the rear, above the pistol grip (takedown pin).
The furniture on military rifles is made from black plastic, hence the nickname “the black rifle”. On the early AR-15 and M16A1 rifles, the handguards were of triangular cross-section, and were made from two non-interchangeable parts. On the M16A2 and later rifles, the handguards are of round cross-section, and have two interchangeable upper/lower sections.

The buttstock on the M16A2 is similar in design to the one of M16A1, but slightly longer. One disadvantage of the Stoner system is that it cannot be adapted for a conventional folding buttstock. Instead, if required, a telescoped stock is used, that allows the rifle to be shortened when required by about half of the length of the standard stock.

The M16 rifle is usually equipped with a sling, and can accept a knife / bayonet, either an old style M7, or a newer style M9. The flash hiders on the earliest AR-15’s and M16’s were prong-type, with three open slots, but were later replaced with “bird-cage” flash hiders with four (M16A1) or five (M16A2) slots.
Both M16A1 and M16A2 can be equipped with an underbarrel 40mm M203 grenade launcher. The M203 mount replaces the standard handguards on the rifle and requires a grenade launcher sight to be mounted on the carrying handle.

Standard sights of the M16A1 consist of a protected front post, mounted on the gas block, and an aperture flip-up rear sight, with 2 range settings. Rear sights are mounted within the carrying handle and are adjustable for windage. The A2 style rear sight also features flip-up, dual aperture sights, with one smaller aperture for daylight usage, and another larger aperture for low light conditions. The range adjustments are made by the rotating knob, located just under the sight. The front sight is generally the same as on the M16A1. The M16A4 rifles have detachable carrying handles with A2 sights, and a Picatinny-type MilStd rail on top of the receiver, that can accept a wide variety of sighting devices and mounts.
The most common military sighting equipment beyond basic iron sights is a Trijicon ACOG low-magnification telescope or Aimpoint or EOTech 1x magnification red-dot sight, often complemented by removable back-up iron sights (BUIS), installed on the same Picatinny rail.




M16A1 M16A2 M16A4
Caliber 5.56x45mm (.223 Remington), M193 5.56x45mm NATO / M855 5.56x45mm NATO / M855
Action gas operated, rotating bolt
Overall length 986 mm 1006 mm 1000 mm
Barrel length 508 mm 508 mm 508 mm
Weight, empty 2.89 kg 3.77 kg 3. 4 kg
Magazine capacity 20 or 30 rounds standard
Rate of fire, cyclic 650 – 750 rounds per minute 700 – 950 rounds per minute